A preferred semiconductor device for power control is the insulated gate FET (Field Effect Transistor) because of its high power gain. FETs used for power switching use are usually enhancement mode types. This means that they are normally non-conducting. When a gate voltage above a threshold is applied, the FET becomes conducting. FETs are available in two gate polarities; N channel and P channel.
Power switching circuits designed for general purpose use are usually constructed with N channel FETs because, for any given die size transistor, the N channel FET has a lower on resistance than a correspondingly sized P channel FET would have.
Present art for radiation hardened power switching circuits use specially designed radiation hardened N-channel FETs for power switching functions. The principal benefit of these radiation hardened N-channel FETs parts is that the gate threshold voltage doesn't change much after being exposed to radiation. However, these parts have limited sources of supply, are expensive and may have long lead times, leading to higher prices and longer delivery times for the radiation tolerant circuits that incorporate these types of parts.
If conventional non-radiation hardened N Channels FETs are used in switching applications where radiation is present, the function tends to fail at relatively low radiation levels because the gate threshold voltage of the N channel FET shifts more negatively with accumulated radiation dose, and ultimately falls close to zero. At this point, the N channel FET conducts current with little or no gate voltage applied. Therefore, the part is difficult to control, since, after a certain amount of accumulated ionizing radiation, the FET drain to source channel will be conducting with no applied power.